Magnetic field control in electrolysis cells

ABSTRACT

An elongated, magnetic flux impeding means is provided in the magnetically conductive shell of an electrolysis cell, and extends in a direction substantially perpendicular to the direction of a magnetic field component intermittent flow of current, in which a saturated mono-or polyvalent ether or a lower alkanoic acid is added to the reaction mixture.

MAGNETIC FIELD CONTROL. [N ELBCTROIIYSIS CELLS Filed Harch 24. 1972 R. F. ROBL Emma 1, 1974 2 Sheets-Sheet 1 MOI Mn. 1, 1974 F, R'QBL 3,783,121

MAGNETIC FIELD CONTROL IN ELECTROLYSIS CELLS Filed March 24, 1972 2 Sheets-Sheet States Patent 3,783,121 MAGNETIC FIELD CONTROL IN ELECTROLYSIS CELLS Robert F. Robl, Monroeville, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa. Filed Mar. 24, 1972, Ser. No. 237,728 Int. Cl. C22d 3/02 US. Cl. 204-243 M 7 Claims ABSTRACT OF THE DISCLOSURE An elongated, magnetic flux impeding means is provided in the magnetically conductive shell of an electrolysis cell, and extends in a direction substantially perpendicular to the direction of a magnetic field component that is induced in the shell by electrolysis current flow in the cell. The flux impeding means is eifective to concentrate the force of the magnetic field component at the location of the flux impeding means, and to generally prevent the saturation of the shell. With the force of the magnetic component concentrated at the location of the flux impeding means, the magnetic field component in the remaining areas of the shell is correspondingly and substantially reduced, and thereby reduced in the adjacent areas of the cell. In addition, a magnetically conductive structure is included to saturate the magnetically conductive shell at critical locations to allow penetration of the shell of a magnetic field component produced externally of the cell.

BACKGROUND OF THE INVENTION The present invention relates generally to electrolysis cells for producing molten metals, such as aluminum, from compounds of the metals, and particularly to a means for substantially reducing the adverse efiects of magnetic fields on the molten metal within the cells.

In the production of aluminum, for example, in an electrolysis cell, molten aluminum is formed by the electrolysis of alumina dissolved in a fused salt bath resulting in a layer of the molten aluminum being formed on the bottom of the cell. It is known that the interaction of magnetic fields and current densities within the cell produce electromotive forces on the molten aluminum layer which results in substantial movement and circulation of the aluminum layer within the cell, which movement causes extensive erosion of the lining. This erosion, particularly at the location of cracks in the lining of the cell, forms pot holes in the lining, which results eventually in the shut down of the cell or cells for the purposes of repairing and/or replacing the lining. As can be appreciated, the shut down of a cell and the repair or replacement of a cell liner is a very costly process since the cell is not producing metal during the time of shut down, and the time, labor and materials for repairing or replacing a liner are considerable.

In addition, the movement of the molten metal layer necessitates an unduly large separation of the cells anodes from the cell liner, which is the cathode of the cell, since the moving layer tends to assume an uneven vertical displacement within the cell beneath the anodes. This large separation of the cells anodes from the cells cathode liner increases the distance for, and thus the electrical resistance to, the flow of current through the cell. This results in an electrically inelficient cell operation through increased voltages between the cathode and anodes and excess power consumption. When the amount of current in each cell is considered, an average value often being on the order of 150,000 amperes, the large number of cells in the potline, the average number often being on the order of 150, it can be appreciated that the total in- 3,783,121 Patented Jan. 1, 1974 crement in the cost of electrical energy can be quite large. When the cost of repairing and replacing cell linings is added to the increase in electrical costs, not controlling the movement of molten metal is very costly.

The magnetic fields causing the movement of the metal layer originate both within the cell and externally of the cell, the major source of the externally produced flux or lines of force originating from bus risers for the cell anodes and from portions of the cathode buses electrically connecting the cell to an adjacent cell. The source of internally produced magnetic flux is the cell current itself, i.e., electrolysis current flowing from the anodes to the cathode liner through the salt bath and the metal layer, and from cathode connector bars extending into the liner.

As a general consideration, the forces that move the metal can be reduced by decreasing the magnetic field strengths within the cell, by decreasing the current densities in the cell and in associated conductor buses, and/ or by rearranging the pattern of these variables in their relationship to the cell and the molten layer of aluminum therein. For example, a prior practice of reducing the adverse influence of magnetic fields within the cell has involved various rearrangements of the cathode and anode leads and buses in their relationship to the cell. It has been found, however, that while reducing one particular magnetic field component within the cell, other magnetic field components have been adversely affected, i.e., the strengths of the other components have been increased or decreased in a manner that actually increases the velocity of the circulating aluminum. This occurs because the electromotive forces that move the molten metal tend to move it in complex circulation patterns so that the mere reduction or elimination of one magnetic field component may increase the problem of metal movement rather than decrease it.

Most cells employed in the aluminum producing industry have an outer, magnetically conductive steel shell, which ordinarily serves to shield the interior of the cell from magnetic flux produced externally of cell. The steel shell, however, is magnetically continuous along the sides and ends of the cell, and completely encloses the cell and collector bar currents. The current flow within the cell generates a magnetic flux that is directed into the magnetically continuous enclosure or shell and thereby induces a high strength magnetic field component in the shell according to Amperes law fH-dl=l (enclosed), i.e., the sum of the magnetic intensities H in the shell, as measured in incremental distances therealong, is equal to the total current enclosed by the shell. For example, assuming a uniform distribution of the magnetic field strength along the shell, the magnetizing force H will sim ply be the enclosed current divided by the distance of the cell circumference. With the increases in electrolysis cell current that have taken place in more recent years, the resultant increase in the strength of .the magnetic fields produced thereby have tended to saturate the shell with magnetic flux, and thereby reduce the eifectiveness of the shell as shield against currents external of the cell.

BRIEF SUMMARY OF THE INVENTION In one embodiment of the invention the force of a magnetic field component induced in the shell of a cell by current flow through the cell, and which component causes the layer of molten metal within a cell to circulate, in cooperation with other field components, is concentrated at critical locations in the shell and cell in such a manner that is is effective to stop or at least substantially slow the movement of metal. This is accomplished by providing the shell with at least one elongated and relatively narrow flux interrupting or impeding area, which area may take the form of a cut or gap in the shell. The cut or gap is preferably filled and closed with a non-magnetic material to seal the cell and to maintain the structural integrity of the shell. The gap in the shell extends in a direction substantially perpendicular to the direction of the magnetic lines of force (flux) produced by cell current. The strength of the magnetic field component in the shell concentrates in the gap and in the vicinity of the gap within the cell where the strength of the component was ordinarily weak and ordinarily did not oppose the movement of the molten metal within the cell. Since the total intensity of the component is fixed (it being dependent upon cell current, as explained above in connection with Amperes law), the intensity of the component in the remainder of the shell, and in the remaining corresponding areas within the cell, is lessened by the amount it is strengthened in the area of the gaps, this lessening of the component also contributing to the slowing and/ or stopping of metal movement.

The cut or gap in the shell, in addition, reduces the tendency of the shell to saturate, thereby increasing its eflectiveness as a shield to magnetic fields produced externally of the cell.

In a further embodiment of the invention, the effect of a helpful field component within the cell is substantially increased by the use of a magnetic field coupling structure. The coupling structure is effective to saturate the cell shell at critical locations to increase the strength or aifect of the externally produced field in the cell that is effective to improve the force pattern acting on the molten metal.

THE DRAWINGS The invenion, along with its advantages and objectives, will be more apparent after considering the following detailed description in connection with the accompanying drawings in which:

FIG. 1 is a partial side elevation view of an electrolysis cell provided with a non-magnetic gap in its outer shell in accordance with the principles of the present invention;

-FIG. 2 is a top view of the cell of FIG. 1; and

FIG. 3 is an enlarged sectional view of the cell of FIG. 1 taken along lines Ell-411.

PREFERRED EMBODIMENTS OF THE INVENTION Referring now to the drawings, FIG. 1 shows an electrolysis cell or pot 10, as it is generally known in industry, for producing a metal, such as aluminum. Such cells or pots comprise generally an outer shell 12 made of a magnetically conductive material, such as steel, and an inner cathode lining 14 made of a carbon material, as indicated in cross section in FIG. 3 and in dash outline in FIG. 1. The shell usually includes an upper deck 16 which may have the configuration shown in FIG. 3 though the invention is not limited thereto.

Extending into the liner 14 of the cell (on both sides thereof) and through openings 17 in the shell 12 are cathode collector bars 18. On the outside of the cell, the collector bars are commonly connected together by collector ring buses (not shown) which, in turn, are connected to further ring buses 20' located at the ends of the cell, such ring buses serially connecting the cell to the anodes of a next adjacent cell in a series thereof generally known in the art as a potline.

Suspended within the cell 10 and within an electrolyte (not shown) within the cell, are anodes 22, the anodes being suspended by electrically conductive rods 24 suitably attached to overhead anode buses (not shown). In a rent flows into the liner and is collected by the bars 18 for conduction to the next cell via buses 20.

As explained earlier, the flow of cell current produces magnetic lines of force (flux) which are directed into the magnetically conductive shell 12, which lines of force tend to saturate the shell and to induce therein a high strength, horizontal magnetic field component, the saturation of the shell greatly decreasing, if not eliminating altogether, its function as a shield to the magnetic fields produced externally of the cell, such as by cathode and anode buses, and by adjacent cells.

The horizontal magnetic field component induced in the shell 12 is reflected in the cell and extends through the layer of molten metal to increase the movement of the metal within the cell. In a cell of the type shown in FIGS. 1 and 2, there exist, in addition to the horizontal component, vertical and transverse field components which, in cooperation with the horizontal component, tend to circulate the metal at relatively high velocities and in complex patterns, one of the more simple flow patterns being that shown in FIG. 2 of the drawings. More particularly, the metal tends to fiow along the ends of the cell away from the center thereof, as indicated by arrows line a in FIG. 2, and horizontally along the sides of the cell toward the center thereof, as indicated by arrow lines b. As the metal moves toward the longitudinal center of the cell it turns toward the transverse center of the cell, as indicated generally by arrow lines 0. In this manner, basic pools of metal flow are generally formed, though as indicated above, metal flow patterns in electrolysis cells employed for the production of metal are complex, and differ among the different type of cell construction, the patterns depending upon such things as the location of collector bars and buses and the proximity of adjacent cells.

Because of the nature of a magnetic field induced by a source of current, such as the main electrolysis current, the strength of the field at the corners of the current source decrease toward zero, whereas at the side centers of the current source, the strength of the field increases to its maximum value. Thus, with shell 12 of FIGS. 1 and 2, the horizontal field component along the sides of the cell 10 near the corners of the cell is rather weak, whereas the strength of the horizontal field increases substantially in the direction of the center side of the cell. This distribution of the strength of the horizontal component exists within the cell 10, and enhances metal movement therein because of the electromotive force it exerts on the metal in a direction perpendicular to the horizontal extent of the field. This electromotive force is exerted on the metal in areas within the cell that assist the flow of metal in the quadrant pattern explained above, i.e., the strength of the horizontal component adjacent the center side of the cell exhibits a force in the direction of arrow lines 0, whereas, in the areas of the cell corners (and arrow lines a) the force of the horizontal field component is weak.

In accordance with the present invention, the strength of the horizontal field component induced in the shell 12 along the center sides of the cell 10 by the magnetic flux generated by cell current, is substantially reduced while the strength of the horizontal component in the cell sides adjacent the corners of the cell are correspondingly and substantially increased (in accordance with Amperes law as explained above). This is accomplished by providing elongated and relatively narrow non-magnetic areas 26 in the shell near the corners of the cell, as shown in the drawing, the areas 26 being effective to interrupt and impede the flow of magnetic flux in the shell, and thereby generally prevent saturation of the shell so that the shell becomes, in addition, an effective shield against magnetic fields generated externally of the cell. With the force of the longitudinal field component concentrated in the nonmagnetic areas 26, and thus in the areas of the cell where the flow of molten metal transverse of the cell is toward the corners, the force exerted on the molten metal by the horizontal component extends toward the center of the cell in direct opposition to the transverse flow of metal. In contrast thereto, the force of the horizontal component along the sides of the cell between the non-magnetic areas 26 is correspondingly weakened so that it tends not to aid the inward flow of metal, as was the case with a magnetically continuous shell.

As shown in FIGS. 2 and 3 of the drawings, the nonmagnetic areas 26 preferably extend into any existing decking 16 of a cell to insure complete interruption of the magnetic circuit provided by the cell shell. As seen in FIGS. 1 and 3, the non-magnetic areas preferably extend from the deck 16 down to the openings 17 in the shell for the two collector bars 18 near the ends of the cell. In FIG. 3, the material of the non-magnetic area 26 is shown in elevation.

The non-magnetic areas 26, as thus far described, may be provided in the shells by making two essentially parallel and narrowly spaced apart cuts therein at each critical location. The narrow portion is then removed to leave an air gap in the shell at the critical location, which gap is suflicient to concentrate the flux and the field component therein. However, in order to prevent the escape of vapors from the cell through the gaps and to otherwise maintain the structural integrity of the cell, the gap or gaps should be filled and sealed with a suitable non-magnetic material. One such material is a non-magnetic stainless steel, which, in the form of a strip or bar is preferably welded in place to the shell, as indicated by heavy lines 27, by the use of a non-magnetic welding material.

With such a shell structure, and with such a concentration and diminution of the force of a magnetic field in critical areas of a cell, as effected by the structure, the movement of molten metal is slowed and controlled in such a manner that the level of the metal within the cell essentially is even and uniform, thereby permitting the anodes of the cell to be located lower in the cell. In this manner, the path for current flow is shortened resulting in lower cell resistance and voltage and considerable savings in the cost of electrical power.

In addition, with the control of metal movement effected by the invention, the life of the liner of the cell is substantially lengthened so that savings are etfected by reducing the frequency of liner repair and replacement.

In the type of cell shown in the drawings, the collector bars 18 extending into the cathode liner 14 generate magnetic flux within the cell having a component that extends at right angles to the flow of current in the bars, and thus vertically through the layer of molten metal within the cell. The magnetic flux generated by each collector bar is influenced by that of the bars adjacent thereto along the length of the cell in a manner that tends to decrease the strength of the resulting vertical field component in the area of the center side of the cell. However, the collector bar located at each corner of the cell has no adjacent bar on the side thereof facing toward the ends of the cell to provide this flux reducing influence, so that in the cell corners the vertical component is quite strong.

In FIGS. 1 and 2 of the drawings, another structure is shown for controlling the movement of molten metal within the cell 10, and thereby effecting further economies of the type described above in connection with the nonmagnetic areas 26. More particularly, in FIGS. 1 and 2, two C-shaped magnetic coupling structures or couplers 30 are shown located about, but spaced from, the end ring buses 20. The couplers are suitably attached to the shell 12 at the cell ends near the downstream side of cell, as seen in FIG. 2, the downstream side of the cell being the side adjacent the next cell receiving current from cell in a potline of serially connected cells.

The couplers are made of a magnetically conductive material, such as iron or steel, and are located adjacent to and about the ring buses 20 in such a manner that the couplers direct and concentrate the magnetic flux produced by the buses, when the buses are conducting electrical current, into the shell 12 of the cell 10 to saturate the same in the area of the couplers. With the shell saturated at the location of the couplers, it is not an elfective shield to the magnetic field produced by the buses 20.

In the operation of cell 10, the cell current collected by the collector bars 18 within the liner 14 of the cell, as explained above, produces a magnetic field component that extends vertically through the cell and through the molten metal in the cell, which components tend to circulate the metal generally in the four basic pools described earlier. With the flow of electrical current in the main ring buses 20, a magnetic field component is produced that extends vertically and in a direction opposite to that of the magnetic field component produced by the collector bars 18 within the cell adjacent its downstream side. With the couplers 30 saturating the shell 12 in area of the couplers so that the shell does not function to shield the interior of the cell from the vertical component produced by the buses 20, the component penetrates into the end corner areas of the cell to substantially subtract from the strength of the vertical component produced by the collector bars within the cell and thus to substantially reduce the velocity of the movement of the molten metal from that caused by the collector bar component. With this reduction in the movement of molten metal, the anodes of the cell can be disposed closer to the liner, thereby reducing anode to cathode resistance and voltage. Further, the wear on the liner is reduced, with the reduction in metal movement, so that liner repair and replacement costs are similarly affected.

In the embodiment of the invention just described, couplers were used to decrease a detrimental magnetic field component within a cell. However, because of the complexity of metal flow patterns within a cell, there are occasions when a magnetic field component existing within a cell can be increased to slow or stop the How of metal, and this can be accomplished with the use of couplers.

From the foregoing description, it can now be appreciated that with large amounts of current flow presently employed in cells, and with the large numbers of cells in a potline for producing metal on a commercial basis, the reductions of voltage and lining wear, as effected by the non-magnetic shell areas 26 and the couplers 30 of the present invention, produce substantial economies in the production of the metal.

While the invention has been described in terms of pre ferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.

Having thus described my invention and certain embodiments thereof, I claim:

1. In combination with an electrolysis cell having a shell made of a magnetically conductive material, said cell, when conducting current therethrough, being a source of magnetic flux which tends to saturate the shell and produce a high strength magnetic field component within said shell in essentially one direction,

elongated, magnetic flux impeding means provided in the shell, and extending in a direction substantially perpendicular to the direction of the magnetic field component, said flux impeding means being effective to concentrate the force of the magnetic field com ponent therein, and within said cell in the area of said flux impeding means, and thereby reduced the magnetic force by a corresponding amount within said cell in the area of the remainder of the shell while simultaneously reducing the saturation of the remainder of the shell and thereby making the shell of the cell an effective shield against magnetic field components produced externally of the cell.

2. The structure of claim 1 in which the flux impeding means comprises an elongated, non-magnetic gap provided in the shell of the cell.

3. The structure of claim 2 in which the magnetic flux impeding means includes an elongated structure made of a non-magnetic material secured in the elongated gap.

4. The structure of claim 1 in which the magnetic field component extends longitudinally within the cell between the ends thereof, and the flux impeding means provided in the shell are located at the sides of the cell adjacent the ends thereof, and extend in a substantially vertical direction.

5. The structure of claim 1 including at least one electrical conductor located externally of and extending along at least a portion of the cell, said conductor, when conducting electrical current, producing a magnetic field extending in a predetermined direction, and

a magnetic coupler made of a magnetically conductive material spaced from and located about at least a portion of said external conductor, and disposed in physical contact with the shell of the cell, said coupler, in cooperation with said conductor when conducting electrical current, being effective to saturate the shell of said cell with magnetic flux at the location of the coupler.

6. In combination with a cell for producing metal by electrolysis, with a layer of molten metal being formed in the cell, said cell having a shell made of a magnetically conductive material, and being effective to shield the interior of the cell from magnetic fields produced externally thereof,

a source of magnetic flux capable of producing a magnetic field component extending in a predetermined direction through said layer of metal,

at least one electrical conductor located externally of and extending along at least a portion of the cell, said conductor, when conducting electrical current, producing a magnetic field extending in a predetermined direction, and

a magnetic coupler made of a magnetically conductive material spaced from and located about at least a portion of said external conductor and disposed in physical contact with the shell of the cell, said coupler, in cooperation with said conductor when conducting electrical current, being effective to increase saturation of the shell of said cell with magnetic flux at the location of said coupler.

7. The combination of claim 6 in which the magnetically conductive coupler has a configuration which, in combination with the shell of the cell, surrounds the external conductor at the location of the coupler.

References Cited UNITED STATES PATENTS 3,640,800 2/1972 Johnson 204244 X 3,616,317 10/1971 McLellan et a1. 204-243 M X 3,385,778 5/1968 Johnson 204-244 X 3,617,454 11/1971 Johnson 204244 X JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner 

